1. Field of the Invention
This invention relates generally to a catalyst for the combination of CO and O.sub.2 to form CO.sub.2. It relates particularly to a catalyst for combining CO and O.sub.2 to form CO.sub.2 in a high-powered, pulsed CO.sub.2 laser.
2. Description of the Related Art
In many applications, it is highly desirable, even necessary, to operate a CO.sub.2 laser in a sealed condition, for in an open system the laser requires a continuous flow of laser gas to remove the dissociation products,--viz., CO and O.sub.2 --that occur in the discharge zone of the laser, in order to maintain a stable power output. This adds to the operating cost of the laser, and in airborne or space applications, it also adds to the weight penalty of the laser. In a sealed CO.sub.2 laser, a small amount of CO.sub.2 gas is decomposed in the electrical discharge zone into corresponding quantities of CO and O.sub.2. As the laser continues to operate, the concentration of CO.sub.2 decreases, while the concentrations of CO and O.sub.2 correspondingly increase. The increasing concentration of O.sub.2 reduces laser power, because O.sub.2 scavenges electrons in the electrical discharge, thereby causing arcing in the electric discharge and a loss of the energetic electrons required to boost CO.sub.2 molecules to lasing energy levels. As a result, laser power decreases rapidly. It is known that the output of a sealed CO.sub.2 laser can be enhanced by the addition of a small amount of water vapor to the laser gas. See W. J Witteman, "The CO.sub.2 Laser", Vol. 53, Springer-Verlag (1987), pp. 104-108. However, this is a short-term effect, after which output starts to fall. Additionally, too much water vapor in the gas will decrease the laser output. See W. J. Witteman, "Increasing continuous laser-action on CO.sub.2 rotational vibrational transitions through selective depopulation of the lower laser level by means of water vapor," Physics Letters, Vol. 18, No. 2, Aug. 15, 1965; and Witteman, W. J., "4B4-Rate determining processes for the production of radiation in high power molecular lasers," IEEE J. of Quantum Electronics, Vol. QE-2, No. 9, September 1966.
A catalyst that can combine CO and O.sub.2 as soon as formed in the laser envelope would prevent the loss of laser power. To be effective, many catalysts must be heated to elevated temperatures above ambient conditions to efficiently combine the products of CO.sub.2 dissociation. This is not feasible in the laser envelope, because the gas in this envelope must be maintained at ambient temperature for efficient operation. This means that the catalyst must be located in a recirculating loop external to the laser; and this requires a pump, a heating system, and a cooling system, all of which can add to operating cost, as well as to weight penalty in airborne or space-borne applications. However, a catalyst effective at ambient operating temperatures can be installed directly in the laser envelope, since the laser gas, comprising CO.sub.2, N.sub.2, and He, is under high circulation at ambient temperature. Many catalysts require either regeneration or replacement, because they degrade with time and lose their activity for combining the CO.sub.2 dissociation products. These requirements not only add to the operating cost and weight penalty of the laser, but also render space-borne applications unfeasible. However, if the ambient-temperature catalyst maintains its activity for long periods of time, a laser can be operated economically in ground-based, airborne, and space-borne applications.
Catalyst formulations of the related prior art consist of (1) a dispersion of colloidal and noncolloidal particles of stannic oxide, either self-supporting or impregnated on an inert support, with a noble metal on the stannic oxide surface (see U.S. Pat. No. 4,524,051) and (2) particles, granules or pellets of stannic oxide with a noble metal on the stannic oxide surface (see European Patent Application No. 83306312.6, dated Oct. 18, 1983).
One of the primary disadvantages of the related prior art is that when the stannic oxide catalyst is in the form of particles, granules, pellets, or supports impregnated with colloidal particles, most of the stannic oxide is below the surface, is structurally bound to other stannic oxide molecules, and is therefore unavailable as active sites. Thus, there is a relatively small surface area of stannic oxide available as active sites for catalytic activity.
Another primary disadvantage of the related prior art is that there is no comprehension of using chemisorbed moisture on a Pt/SnO.sub.2 catalyst surface to enhance and prolong its activity.